1. Field of the Invention
The present invention relates to a light-emitting element and an optical device and more particularly to a light-emitting element for emitting light in a near-infrared region formed on a III-V group compound semiconductor substrate and an optical device.
2. Description of the Related Art
Light in a near-infrared region is used in texture observation of animals and plants, communication, and night photography. Thus, near-infrared light-receiving elements and light-emitting elements are being studied and developed. In particular, regarding light-emitting elements, InGaAsP light-emitting diodes (LEDs) for use in optical communication wavelength regions and near-infrared light-emitting diodes having an emission wavelength of 2.0 μm or more formed on a GaSb substrate are commercially available (Kyosemi Corporation, catalog, InGaAsP infrared LED KEDE1542H and Keystone International Co., Ltd., catalog, NEW MID INFRARED PRODUCTS).
Light-emitting elements having a multi-quantum well (MQW) structure mainly have a type 1 multi-quantum well structure. However, it is suitable for near-infrared light-emitting elements to have a type 2 MQW structure. Because of its high transition probability, light emission in the type 1 multi-quantum well structure has sufficient luminescence intensity even when the active layer has a small thickness or a small number of pairs. In the case of light emission in the type 2 multi-quantum well structure, two types of semiconductors having a large band gap can be used to make a transition for emitting near-infrared light corresponding to a narrow band gap. Since the transition probability is low, however, the number of pairs must be increased to achieve sufficient luminescence intensity. An increase in the number of pairs results in the accumulation of defects in the quantum well, making it difficult to achieve high crystallinity.
M. Peter, et al. discloses an active layer composed of a type 2 multi-quantum well structure of 10 to 20 InGaAs/GaAsSb pairs as an example of InP compound semiconductor light-emitting diodes (LEDs) and laser diodes (LDs) (M. Peter, et al., “Light-emitting diodes and laser diodes based on a Ga1-xInxAs/GaAs1-ySby type II superlattice on a InP substrate”, Appl. Phys. Lett., Vol. 74, No. 14, 5 Apr. 1999). The number of pairs is limited because of the reason described above. An epitaxial layered body is grown on an InP substrate by metal-organic vapor phase epitaxy (MOVPE).
H. Takasaki, et al., reported the observation of electroluminescence having a center wavelength of 2.4 μm from an InGaAs/GaAsSb type 2 multi-quantum well structure at room temperature (H. Takasaki, et al., “Electroluminescence of In0.53Ga0.47As/GaAs0.5Sb0.5 type II multiple quantum well diodes lattice-matched to InP”, Journal of Crystal Growth, 227-228 (2001), 294-297).
Because of their narrow bands and low intensities of light, the LEDs and other devices described above are not used in practical analytical apparatuses (optical devices). For example, halogen lamps and supercontinuum (SC) light sources are used for analyses of proteins having a plurality of absorption bands. Although halogen lamps can emit light in a wide near-infrared region, they generate excessive heat and alter the quality of the target organic substances, such as proteins. Supercontinuum light sources can emit light in a wide wavelength range based on a non-linear phenomenon by supplying single-wavelength light having a very high energy density through an optical fiber. However, supercontinuum light sources are expensive.